To enforce martial law on the USA would take an army of 20-30 million. Of coarse some states/regions would be on the side of the USA and some not.
The Germans maintained around 100,000 troops in occupied France shortly after the surrender. At the time France had a population around 37,800,000 and was occupied by ~100,000 Germans.
Some suggest to occupy/control a nation you need between 5 to 50 troops per 100 of population. That'd require somewhere between 6 million and 66 million troops....give or take.
You're correct...at this time we don't have those numbers.
And I hope we never do.